Align heading support with Stack Exchange

[b-kelly]

Reported on Meta.SE.

Is your feature request related to a problem? Please describe.

This markdown:

# one heading

## two heading

### three heading

#### four heading

##### five heading

###### six heading

renders this preview in the rich editor:

It gets posted like such:

Describe the solution you'd like

Fix the rendering in the editor such that h4-h6 are not shown as headings (or add h4-h6 support to SE, but that's out of scope here).

[aaronshekey]

My advice would be to allow the full HTML spec of headers on Stack Overflow. I don't share the opinion that posts shouldn't have h4–h6 ✌️ We style 'em. Let 'em have it!

[b-kelly]

Fixed. SE's server-side commonmark implementation now allows up to h6, so we don't have to change anything here.